The current industrial standard processes for front side metallization of silicon photovoltaic cells are based on printing of silver pastes or inks that need to be fired for good contact formation. Traditional screen printing of silver pastes is limited to emitters having a sheet resistance in the order of 60 to 90 Ohm/square, with an emitter surface concentration above 1020 at/cm3 to allow sufficient Ag crystallite formation.
Due to their excellent contact properties, nickel silicide layers can be used for forming good contacts on lowly doped emitters with a surface concentration lower than 1020 at/cm3. Nickel silicide layers can be formed by providing a thin nickel layer (e.g. by electroless plating) on a silicon surface, followed by an annealing or sintering step to induce silicidation, resulting in the formation of a nickel-silicon alloy (nickel silicide). The annealing step is typically done in an inert environment (e.g. N2) by rapid thermal annealing (RTA) or in a belt firing furnace.
The main challenge related to nickel silicide formation on top of an emitter junction is avoiding shunting of the emitter junction. Shunting may result from nickel diffusion into the silicon during the silicidation process.
In a process for fabricating photovoltaic cells, a patterned nickel silicide layer can be formed at the front side of the cells in openings created in an antireflection coating. On top of the nickel silicide layer at least one additional metal layer (such as a Cu layer) is typically electroplated (using the nickel silicide layer as a seed layer) to form low resistance contact paths. In an industrial fabrication process, the openings in the antireflection coating are typically formed by laser ablation, e.g. using a ps UV laser. Such a laser ablation step creates defects or damage to the silicon surface, enhancing the risk of nickel diffusion during a subsequent silicidation process and thus an increased risk of emitter shunting, e.g. due to nickel spiking.
On textured silicon surfaces, increased laser ablation occurs at the pyramid tips and edges, as reported by A. Knorz et al in “Selective Laser Ablation of SiNx Layers on Textured Surfaces for Low Temperature Front Side Metallizations”, Prog. Photovolt: Res. Appl. 2009, 17, 127-136. It was shown that under irradiation with laser light with a wavelength of 355 nm a textured surface causes local amplifications of the electromagnetic field, leading to an inhomogeneous irradiation of the surface and therefore preventing a completely damage free laser ablation of the antireflection coating.